1. Field of the Invention
This invention relates to antennas having frequency reuse capabilities, and more particularly to antennas having a four port network or quadruplexer located in the antenna waveguide, a feed horn attached to the waveguide, and a polarizer disposed at the aperture of the antenna for converting linearly polarized signals to circularly polarized signals.
2. Description of the Prior Art
It has become well known in the field of satellite communications to utilize a single antenna to transmit and receive signals in two frequency bands with two orthogonal, linearly polarized signal components within each band. Waveguides that incorporate such features are known as four-port networks and/or quadruplexers. U.S. Pat. No. 4,630,059 issued to Morz on Dec. 16, 1986 teaches a four-port network suitable for satellite communication. Two orthogonal ports of the Morz waveguide are utilized to introduce orthogonal linearly polarized signals in the four GHz band which are converted to circularly polarized signals in the throat of the waveguide for transmission through the grooved conical horn. Two other orthogonally disposed ports are arranged to receive linearly polarized signals in the six GHz band.
Another prior art four port waveguide network antenna has been designed by COMSAT Laboratories. This device includes two coaxial waveguides, the outer waveguide being used for the transmission and reception of the four GHz band and the inner coaxial waveguide being utilized for the six GHz band. A tunable configuration of screws and baffles within the waveguides are utilized to convert the linearly polarized signals into circularly polarized signals. The device utilizes a grooved conical horn to transmit and receive signals.
Additional prior art antennas that are of interest include those described in U.S. Pat. No. 4,797,681 to Kaplan et. al. on Jan. 10, 1989; U.S. Pat. No. 4,707,702 issued to Withers on Nov. 17, 1987; U.S. Pat. No. 4,573,054 issued to Bouko et. al. on Feb. 25, 1986; U.S. Pat. No. 4,358,770 issued to Satoh et. al. on Nov. 9, 1982; U.S. Pat. No. 4,219,820 issued to Crail on Aug. 26, 1980 and U.S. Pat. No. 3,898,667 issued to Raab on Aug. 5, 1975.
The efficiency of a satellite antenna which transmits and receives different information utilizing orthogonal polarizations of the same frequency band depends to a significant measure upon the elimination of cross-polarization between the orthogonal polarized signals. It is known that a circularly polarized signal can be reduced to a linearly polarized signal utilizing a meanderline polarizer. Such meanderline polarizers produce minimal cross-polarization and therefore promote efficiency. U.S. Pat. No. 3,754,271 issued to Epis on Aug. 21, 1973 describes a meanderline polarizer having a plurality of stacked substantially identical arrays of laterally spaced square-wave shaped meanderlines. The device is positioned at the aperture of a pyramidal horn for conversion of circularly polarized waves into linearly polarized waves.